Solid polymer electrolytes (SPEs) containing dissolved metal salts have been proposed as alternatives to liquid electrolytes in electrochemical systems. There are many advantages to using a solid electrolyte, such as the capability for high speed production of thin cells constructed in a bipolar configuration. Further, the polymer electrolyte can act as a mechanical barrier between the anode and cathode thereby eliminating the need for an inert porous separator as well as acting as a binder/adhesive to move and conform to electrode volume changes during cycling. The polymer electrolytes also allow and facilitate the fabrication of cells in any geometric shape and also provide an inherent safety advantage over liquid electrolytes since there is no liquid component in the cell to leak out if the integrity of the sealed cell is broken.
One of the most commonly used polymer electrolytes is based on high molecular weight polyethylene oxide (PEO). An ionically conducting solid polymer electrolyte can be prepared by dissolving PEO and an appropriate salt such as lithium perchlorate (LiClO.sub.4), lithium tetrafluoroborate (LiBF.sub.4), lithium trifluoromethanesulfonate (LiCF.sub.3 SO.sub.3) or lithium hexafluoroarsenate (LiAsF.sub.6) in a suitable volatile solvent such as acetonitrile (CH.sub.3 CN). By solution casting, acetonitrile is removed by evaporation, leaving a free standing solid, flexible film of good mechanical strength that contains only PEO with dissolved salt. Such films are ionic conductors.
However, because of the poor ionic conductivity of these polymers of about 10.sup.-7 S/cm, these polymers are not practical as electrolytes for electrochemical cells and particularly, rechargeable cells.